Suppose you want to debug a application already running on a remote machine inside container. And would like to do it without modifying command arguments (enabling mode) and opening remote Node.js debugger agent port to the whole world. Node.js Docker debug I bet you didn’t know that it’s possible and also have no idea how to do it. I encourage you to continue reading this post if you are eager to learn some new cool stuff. The TdodoMVC demo application I’m going to use a of Node.js application (by Gleb Bahmutov) as a demo application for this blog post. Feel free to clone and play with this repository. fork TodoMVC Here is the , I’ve added, for TodoMVC application. It allows to run TodoMVC application inside a Docker container. Dockerfile FROM alpine:3.5 # install nodeRUN apk add --no-cache nodejs-current tiniRUN mkdir -p /usr/src/appWORKDIR /usr/src/app # Build time argument to set NODE_ENV (‘production’’ by default)ARG NODE_ENVENV NODE_ENV ${NODE_ENV:-production} # install npm packages: clean obsolete filesCOPY package.json /usr/src/app/RUN npm config set depth 0 && \npm install && \npm cache clean && \rm -rf /tmp/* # copy source filesCOPY . /usr/src/appEXPOSE 3000 # Set tini as entrypointENTRYPOINT [“/sbin/tini”, “--“] CMD [ “npm”, “start” ] # add VCS labels for code sync and nice reportsARG VCS_REF=”local”LABEL org.label-schema.vcs-ref=$VCS_REF \org.label-schema.vcs-url="https://github.com/alexei-led/todomvc-express.git" Building and Running TodoMVC in a Docker container: To build a new Docker image for TodoMVC application, run the command. docker build $ # build Docker image; set VCS_REF to current HEAD commit (short)$ docker build -t local/todomvc --build-arg VCS_REF=`git rev-parse ---short HEAD` .$ # run TodoMVC in a Docker container$ docker run -d -p 3000:3000 --name todomvc local/todomvc node src/start.js The Plan — I would like to be able to attach a Node.js debugger to a Node.js application already up and running inside a Docker container, running on remote host machine in AWS cloud, without modifying the application, container, container configuration, or restarting it with additional flags. Imagine that the application is running and there is some problem happening right now – I want to connect to it with debugger and start looking at the problem. The final goal debug So, I need a plan — a step-by-step flow that will help me to achieve the final goal. Let’s start with exploring the inventory. On the server (AWS EC2 VM) machine, I have a Node.js application running inside a Docker container. On the client (my laptop), I have an IDE (Visual Studio Code, in my case), Node.js application code ( ), and a Node.js debugger. git pull/clone So, here is my plan: Set already running application to mode debug Expose a new Node.js debugger agent port to enable remote debugging in a secure way Syncronize client-server code: both should be on the same commit in a tree git Attach a local Node.js debugger to the Node.js debugger agent port on remote server and do it in a secure way And, if everything works, I should be able to perform regular debugging tasks, like setting breakpoints, inspecting variables, pausing execution and others. Step 1: set already running Node.js application to the mode debug The V8 debugger can be enabled and accessed either by starting Node with the _--debug_ command-line flag or by signaling an existing Node process with _SIGUSR1_ . (Node API documentation) So, in order to switch on Node debugger agent, I just need to send the signal to the Node.js process of TodoMVC application. Remember, it’s running inside a Docker container. What command can I use to send process signals to an application running in a Docker container? Cool! SIGUSR1 The command – is my choice! This command does not actually “kill” the process, running in a Docker container, but sends a to it (by default it sends ). docker kill PID 1 Unix signal SIGKILL Setting TodoMVC into mode debug So, all I need to do is to send to my TodoMVC application running inside Docker container. SIGUSR1 todomvc There are two ways to do this: Use command to send to process running inside Docker container, and if it’s a “proper” (signal forwarding done right) init application (like ), than this will work docker kill --signal SIGUSR1 PID 1 tini Or, execute inside already running Docker container, sending signal to the main Node.js process. kill -s SIGUSR1 SIGUSR1 $ # send SIGUSR1 with docker kill (if using proper init process)$ docker kill --signal SIGUSR1 todomvc$ # OR run kill command for node process inside todomvc container$ docker exec -it todomvc sh -c ‘kill -s SIGUSR1 $(pidof -s node)’ Let’s verify that Node application is set into mode. debug $ docker logs todomvc TodoMVC server listening at emitting 2 todos server has new 2 todosGET / 200 31.439 ms — 3241GET /app.css 304 4.907 ms— —Starting debugger agent.Debugger listening on 127.0.0.1:5858 http://:::3000 As you can see the Node.js debugger agent was started, but it can accept connections only from the , see the last output line: localhost Debugger listening on 127.0.0.1:5858 Step 2: expose Node debug port In order to attach a remote Node.js debugger to a Node application, running in the mode, I need: debug Allow connection to debugger agent from any (or specific) IP (or IP range) Open port of Node.js debugger agent outside of Docker container How to do it when an application is already running in a Docker container and a Node.js debugger agent is ready to talk only with a Node.js debugger running on the same machine, plus a Node.js debugger agent port is not accessible from outside of the Docker container? Of course it’s possible to start every Node.js Docker container with exposed debugger port and allow connection from any IP (using special and Node.js flags), but we are not looking for easy ways :). --debug-port --debug It’s not a good idea from a security point of view (allowing unprotected access to a Node.js debugger). Also, if I restart an already running application with debug flags, I’m going to loose the current execution context and may not be able to reproduce the problem I wanted to debug. I need a better solution! Unfortunately, Docker does not allow to expose an additional port for already running Docker container. So, I need somehow to connect to a running container network and expose a new port for Node.js debugger agent. Also, it is not possible to tell a Node.js debugger agent to accept connections from different IP addresses, when Node.js process was already started. Both of above problems can be solved with help of small the Linux utility called (SOcket CAT). This is just like the but with security in mind (e.g., it support chrooting) and works over various protocols and through files, pipes, devices, TCP sockets, Unix sockets, a client for SOCKS4, proxy CONNECT, or SSL etc. socat netcat From man page: socat _socat_ is a command line based utility that establishes two bidirectional byte streams and transfers data between them. Because the streams can be constructed from a large set of different types of data sinks and sources (see address types), and because lots of address options may be applied to the streams, _socat_ can be used for many different purposes. Exactly, what I need! So, here is the plan: I will run a new Docker container with the utility onboard, and configure Node.js debugger port forwarding for TodoMVC container. socat : socat.Dockerfile FROM alpine:3.5RUN apk add --no-cache socatCMD socat -h Building socat Docker container $ docker build -t local/socat — < socat.Dockerfile Allow connection to Node debugger agent from any IP I need to run a “sidecar” container in the same network namespace as the container and define a port forwarding. socat todomvc $ # define local port forwarding$ docker run -d --name socat-nip --network=container:todomvc \local/socat socat TCP-LISTEN:4848,fork TCP:127.0.0.1:5858 Now any traffic that arrives at port will be routed to the Node.js debugger agent listening on . The port can accept traffic from any IP. It’s also possible to use an IP range to restrict connection to the listening port, adding option. 4848 127.0.0.1:5858 4848 socat range=&lt;ANY IP RANGE&gt; Exposing Node.js debugger port from Docker container First, we will get IP of Docker container. todomvc $ # get IP of todomvc container$ TODOMVC_IP=$(docker inspect -f “{{.NetworkSettings.IPAddress}}” todomvc) Then, configure port forwarding to the “sidecar” port, we define previously, running on the same network as the container. socat todomvc $ # run socat container to expose Node.js$ # debugger agent port forwarder$ docker run -d -p 5858:5858 --name socat local/socat \socat TCP-LISTEN:5858,fork TCP:${TODOMVC_IP}:4848 Any traffic that will arrive at the port on the Docker host will be forwarded, first, to the socat port and then to the Node.js debugger agent running inside the Docker container. 5858 4848 todomvc Exposing Node.js debugger port for remote access In most cases, I would like to debug an application running on a remote machine (AWS EC2 instance, for example). I also do not want to expose a Node.js debugger agent port unprotected to the whole world. One possible and working solution is to use SSH tunneling to access this port. $ # Open SSH Tunnel to gain access to servers port 5858.$ # Set `SSH_KEY_FILE` to ssh key location or add it to ssh-agent$ #$ # open an ssh tunnel, send it to the bg, and wait 20 seconds$ # for connections, once all connections are closed$ # after 20 seconds then close the tunnel$ ssh -i ${SSH_KEY_FILE} -f -o ExitOnForwardFailure=yes \-L 5858:127.0.0.1:5858 ec2_user@some.ec2.host.com sleep 20 Now all traffic to the will be tunneled over to the remote Docker host machine and after some forwarding to the Node.js debugger agent running inside the container. localhost:5858 SSH socat todomvc Step 3: Synchronizing on the same code commit In order to be able to debug a remote application, you need to make sure that you are using the same code in your IDE as one that is running on remote server. I will try to automate this step too. Remember the command, I’ve used in TodoMVC ? LABEL Dockerfile These labels help me to identify git repository and commit for the application Docker image: – contains short SHA for a commit org.label-schema.vcs-ref HEAD – contains an application git repository URL (I can use in ) org.label-schema.vcs-url clone/pull I’m using (Label Schema Convention)[http://label-schema.org/rc1/], since I really like it and find it useful, but you can select any other convention too. This approach allows me, for each, properly labeled, Docker image, to identify the application code repository and the commit it was created from. $ # get git repository url form Docker image$ GIT_URL=$(docker inspect local/todomvc| jq -r ‘.[].ContainerConfig.Labels.”org.label-schema.vcs-url”’)$ # get git commit from Docker image$ GIT_COMMIT=$(docker inspect local/todomvc| jq -r ‘.[].ContainerConfig.Labels.”org.label-schema.vcs-ref”’)$$ # clone git repository, if needed$ git clone $GIT_URL$ # set HEAD to same commit as server$ git checkout $GIT_COMMIT Now, both my local development environment and remote application are on the same git commit. And I can start to debug my code, finally! Step 4: Attaching local Node.js debugger to debugger agent port To start debugging, I need to configure my IDE. In my case, it’s and I need to add a new configuration. Visual Studio Code Launch This launch configuration specifies remote debugger server and port to attach and remote location for application source files, which should be in sync with local files (see the previous step). { // For more information about Node.js debug attributes, visit: “version”: “0.2.0”,“configurations”: [ {“type”: “node”,“request”: “attach”,“name”: “Debug Remote Docker”,“address”: “127.0.0.1”,“port”: 5858, “localRoot”:“${workspaceRoot}/”,“remoteRoot”: “/usr/src/app/” }] } https://go.microsoft.com/fwlink/?linkid=830387 Summary And finally, I’ve met my goal: I’m able to attach a Node.js debugger to a Node.js application, that is already up and running in a Docker container on a remote machine. It was a long journey to find the proper solution, but after I found it, the process does not look complex at all. Now, once I meet a new problem in our environment I can easily attach the Node.js debugger to the running application and start exploring the problem. Nice, isn’t it? I’ve recorded a short movie, just to demonstrate all steps and prove that things are working fluently, exactly as I’ve described in this post. Hope, you find this post useful. I look forward to your comments and any questions you have. Originally published at codefresh.io on June 6, 2017. is how hackers start their afternoons. We’re a part of the family. We are now and happy to opportunities. Hacker Noon @AMI accepting submissions discuss advertising & sponsorship To learn more, , , or simply, read our about page like/message us on Facebook tweet/DM @HackerNoon. If you enjoyed this story, we recommend reading our and . Until next time, don’t take the realities of the world for granted! latest tech stories trending tech stories

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